The present invention relates to optical communications networks. In particular, this invention relates to the use of a passive tree-and-branch optical fiber network that employs Time Division Multiple Access (TDMA) in a telephone subscriber loop environment.
In the past ten years optical transmission systems have more and more taken over the functions of their copper counterparts in the trunk network between central offices. However, plain replacement of copper based transmission in the trunk network by optical fiber based transmission is only the first step in the utilization of the large transmission capabilities of optical fibers. Indeed, deployment of optical fibers will penetrate further to the local loop plant to bring broad-band and narrow-band services directly to customers.
Currently, one area of investigation in optical loop plants involves the use of a passive optical network in which physical access to the medium is controlled by TDMA techniques and information transfer takes place using the Asynchronous Transfer Mode (ATM) transmission format. This passive optical network uses a tree-and-branch network topology to implement the local loop which interconnects customers to the central office. In particular, each customer station, or optical network termination (ONT) node, is connected via the same optical fiber to an optical line termination (OLT) node of the central office. The OLT of the central office centrally controls the information flow to, and from, the various ONTs using TDMA techniques. In particular, the OLT assigns TDMA time slots, which are used by the ATM transmission format. Information that flows from the OLT to each of the various ONTs is considered to be a "downstream" information flow and, conversely, information that flows from each ONT to the OLT is considered to be an "upstream" information flow.
Generally speaking, the ATM transmission format is based on a frame of information, where each frame includes a number of "cells" and where each ATM cell carries specific information within a particular TDMA time slot. Each ONT receives the entire downstream ATM frame via a representative optical signal transmitted from the OLT. However, the ATM transmission format allows for ATM cells to be individually assigned to an ONT so that even though each ONT receives the entire ATM frame, it only selects those ATM cells to which it is assigned and ignores the others. In addition, each ONT synchronizes its clock to the received optical signal.
In the upstream direction, as mentioned above, each ONT similarly transmits information in its assigned ATM cells under the control of the OLT of the central office. Unfortunately, this situation is more complicated because, although this is a subscriber loop network where the distance between each ONT and the OLT is relatively short, the transmit time of information traveling through the optical fiber is not negligible. Consequently, in order to prevent collisions at the OLT, each ONT not only has to know which TDMA time slot to put the ATM cells into but also must know when to put its information-bearing ATM cells onto the network. In other words, each ONT must compensate for the different optical path lengths in the network. This synchronization of the ONTs is accomplished by a "ranging function" implemented within the OLT. This ranging function estimates the distance, or time delay, from the OLT to each ONT.
The ranging procedure to get all ATM cells in line at the OLT side can be further divided into "static ranging" and "dynamic ranging." Static ranging is usually done initially and every time when a new customer is connected to the network (and automatically in case of a failure). This static ranging is generally done in two steps: "coarse" ranging yielding a resolution of a few ATM cells and "fine" ranging yielding a resolution of a few bits. Dynamic ranging is usually performed (quasi) continuously to overcome delay variations in the optical network and electronics, e.g., due to temperature changes. The goal of the whole ranging process is to obtain virtually identical OLT-ONT distances for each ONT in the network. Once differences in OLT-ONT distances are determined in the static ranging procedure and delay times have been installed at each of the ONTs, upstream information transfer is allowed to begin. A dynamic fine ranging procedure is usually done by monitoring the gaps between received ATM cells at the OLT side. The delay is adjusted when the gap between ATM cells becomes too large or too small.
One method to determine the range from the OLT to each ONT is to create a large idle TDMA time slot in which no transmission of user information can take place. In particular, the OLT sends a ranging instruction, or message, to a particular ONT, which then immediately sends back a ranging cell to the OLT. The OLT measures the delay between when it sent the ranging instruction and the ONT's response and calculates the distance to that particular ONT and therefrom the amount of time delay to be installed at that ONT. During this ranging procedure, all the other ONTs must remain silent, i.e., they cannot transmit any information.
Another method to determine the range from the OLT to each ONT makes use of auto-correlation techniques and is performed in two steps. In the first step, a coarse ranging estimate is determined by the use of a superimposed low-level, low frequency, digital ranging signal on the high frequency bit stream to the OLT. This digital ranging signal represents a digital sequence, or bit pattern, that is recognizable by the OLT. At the start of the ranging procedure, the OLT requests the ONT to transmit the digital ranging signal. On reception of this command, the ONT immediately transmits the predefined digital ranging signal upstream to the OLT. Auto-correlation techniques and an automatic zero control circuit are used to allow recognition of the low-level digital sequence. Ranging inaccuracy obtained with this method is typically a few ATM cells.
The second step of this method performs fine ranging in accordance with the first method mentioned above. In particular, upstream transmission is briefly interrupted and large idle TDMA time slots are created to transmit ranging information. Fine ranging reduces the delay uncertainty to a few bits.
Unfortunately, both of the above-mentioned methods are not completely attractive solutions to the ranging problem. For example, the first method interrupts the transmission of information from the ONTs to the OLT of the central office. In particular, when creating a large idle TDMA time slot to carry ranging information, all ONTs connected to the network have to remain silent in order to avoid collisions with the ranging cell of the ONT to be ranged. As a result, additional buffers are needed at each of the ONTs to store information while the ranging process is taking place. In addition, during the quiet period a number of ATM cells also have to be buffered at the OLT side. Consequently, depending on how frequently this ranging procedure has to be executed, transmission capacity in both the downstream or the upstream direction can be seriously decreased. Further, the use of idle TDMA time slots in the downstream direction makes it more difficult to recover the clock signal at the ONTs. Finally, while the second method minimizes the interruption of the information flow, the use of auto-correlation techniques in the first step of the second method is a complex process that may require dedicated and expensive hardware to implement the coarse ranging function.